1. Technical Field
The present invention relates to a sound transmitter for radiating a sound wave in water. In particular, the present invention relates to a bending vibration type sound transmitter for radiating a sound wave in water using bending vibration of a vibrating body.
2. Background Art
In an underwater survey such as an oceanographic survey, various sound transmitters using a sound wave as an observation tool are employed. A sound wave having a low frequency is often used because of low attenuation and good transmission characteristics in water. In the sound transmitters, in order to emit a sound wave with a low frequency efficiently, a vibrating body which generates the sound wave using a mechanical resonance as a radiation source thereof is often used.
An example of a vibration form of the vibrating body which generates the sound wave using the mechanical resonance is shown in FIG. 13. In FIG. 13, both ends of the vibrating body 23 are fixed on edge of fixing members 24. In such condition, as shown by dotted lines in FIG. 13, the vibrating body 23 resonates so that boundaries between the vibrating body 23 and the edge fixing members 24 become supporting points for forming nodes of vibration. When a full length of the vibrating body 23 is L0, and a length (free length) of a part of the vibrating body 23 which is not fixed on the edge of fixing members 24 is L1, resonant vibration of the vibrating body 23 generates a sound wave with a half-wave length of the free length L1. Therefore, in order to generate a sound wave with a low frequency, the free length L1 of the vibrating body 23 needs to be long.
The vibrating body 23 resonates so that boundaries between the vibrating body 23 and the edge fixing members 24 become supporting points for nodes of vibration. In order to maintain the stable resonant vibration, a fixed length L2 needs to be long to some extent so that the nodes of the resonant vibration do not move. When the free length L1 of the vibrating body 23 is long, it is desirable also to make the fixed length L2 long.
On the other hand, the sound transmitter is usually required to be small and lightweight. But, when the free length L1 and the fixed length L2 of the vibrating body 23 are made long to lower a resonance frequency, the full length L0 of the vibrating body 23 becomes long and a weight thereof increases. Therefore, the sound transmitter becomes large and heavy.
Here, as the sound transmitter using a sound wave with a low frequency, a bending vibration type sound transmitter is widely used. The bending vibration type sound transmitter includes a vibrating body in which a driving transducer and a vibrating plate for sound wave emission are arranged. The driving transducer is a piezoelectric transducer which vibrates by applying a voltage signal. The vibrating plate for sound wave emission includes a metallic thin plate or the like. The driving transducer makes bending vibration of the vibrating plate to generate a sound wave. Moreover, the metallic thin plate has low elasticity and can lower the resonance frequency of the vibrating body. Therefore, a small sound transmitter for generating a sound wave with a low frequency can be realized.
A bending vibration type sound transmitter is disclosed in Japanese Patent Application Laid-Open No. 1991-11898 (document 1). In the bending vibration type sound transmitter, a cylindrical metal body having a plurality of slits in an axial direction thereof forms a plurality of vibrating plates (vibrating plates for sound wave emission). The piezoelectric transducers (driving transducers) are used on inside or outside surfaces of the vibrating plates. In the bending vibration type sound transmitter, the piezoelectric transducers are vibrated by application of a voltage signal. The vibrating plates perform bending vibration according to vibration of the piezoelectric transducers and generate a sound wave.
Other bending vibration type sound transmitter is disclosed in Japanese Patent Gazette No. 3520837 (document 2). In the bending vibration type sound transmitter of the document 2, a cylindrical vibrator (driving transducer) is formed of laminated and combined vibrator pieces. The cylindrical vibrator includes a plurality of side face vibrating plates (first vibrating plates for sound wave emission) and end face vibrating plates (second vibrating plates for sound wave emission). Here, a plurality of slits formed in an axial direction of a cylindrical metal plate form the side face vibrating plates therein. In the bending vibration type sound transmitter, the cylindrical vibrators vibrate according to application of a voltage signal. The side face vibrating plates and the end face vibrating plates perform bending vibration to generate a sound wave. Since two kinds of vibrating plates, the side diaphragms and the end face vibrating plates, are used as vibrating plates for sound wave emission, a frequency of a sound wave is lowered and a frequency band is broadened.
Moreover, other bending vibration type sound transmitter is disclosed in Japanese Patent Gazette No. 3649151 (document 3). In the bending vibration type sound transmitter of the document 3, a plurality of vibrating plates (vibrating plates for sound wave emission) are divided by a plurality of slits formed in an axial direction of a cylindrical elastic body. Each end of the upper part and the lower part of the plurality of vibrating plates is fixed on each of two vibrating bodies (driving transducers). The two vibrating bodies vibrate to make bending vibration of a plurality of vibrating plates and generate a sound wave.
Here, in the bending vibration type sound transmitters disclosed in the documents 1-3, the plurality of vibrating plates are formed by the plurality of slits in an axial direction of the cylindrical body. A fragmentary perspective view of an example of a side piece of the cylindrical body is shown in FIG. 14. In FIG. 14, a part where a slit 25 of the cylindrical body is formed performs bending vibration by using a boundary of a part where a slit 25 of the cylindrical body is not formed as a supporting point on which a node of bending vibration is formed. Therefore, in order to generate a sound wave of a low frequency, the length L1 of the part where the slit 25 is formed should be made long. On the other hand, in order to maintain the bending vibration of the cylindrical body in the stable state as mentioned above, the length L2 of the part where the slit 25 of the cylindrical body is not formed needs to be made long to some extent so that the supporting point of bending vibration does not move.